The present invention relates to devices having magnetoresistive material and, more particularly to devices having magnetoresistive material comprising two-phase metallic ferromagnetic components which exhibit the giant magnetoresistance (GMR) effect.
A typical measure of magnetoresistance is given by
xcex94xcfx81/xcfx81=(xcfx810xe2x88x92xcfx81H sat)/xcfx81Hsat,
where xcfx81Hsat is the resistivity of the material when the applied magnetic field is at the saturation value Hsat and xcfx810 is the resistivity of the material when the applied magnetic field is 0. The GMR effect, where the magnetoresistance ratio, xcex94xcfx81/xcfx81, is greater than a fraction of a percent, was first discovered in multilayered thin film structures and subsequently in metallic films containing magnetic particles (magnetic granular films). More recently, large xcex94xcfx81/xcfx81 has been observed in a device called a magnetic tunnel junction (MTJ), in which two ferromagnetic electrodes are separated by an insulating layer that is thin enough to permit quantum mechanical tunneling between the two electrodes. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations of the spin polarizations of the two electrodes. Although thin film GMR and MTJ devices are useful for sensing magnetic field and other applications involving small scales, such as sensing magnetically stored information in a computer disk memory, they are not suited to large scale applications such as in anti-lock braking systems for vehicles and rotary machine feedback systems, which require bulk quantities, sheets or thick films of magnetoresistive material. Moreover, such large scale applications typically require the device to operate at normal ambient temperatures, such as room temperature (300xc2x0 K).
U.S. Pat. No. 5,856,008 to Cheong et al. describes forming a magnetoresistive material consisting of compacted CrO2 powder with the grains of the powder at least partially coated with a thin layer of insulating Cr2O3. Although the Cheong et al. patent describes this material as having a magnetoresistance ratio of greater than 12%, such a high magnetoresistance ratio was achieved only at a cryogenic temperature of 5xc2x0 K and a relatively high magnetic field of 20,000 Oe. The data contained in the Cheong et al. patent shows that the material has a magnetoresistance ratio of 0.2% at 200xc2x0 K and 1000 Oe, and there is no data in the patent showing that the material has any GMR effect at normal ambient temperatures, such as room temperature (300xc2x0 K).
The Cheong et al. patent provides an explanation for the desirable magnetoresistance ratio as being attributable to spin-polarlized tunneling between grains, with the insulating material, i.e., Cr3O2, enhancing the spin-polarization effect, and cites the references J. Inoue et al., xe2x80x9cTheory of Tunneling Magnetoresistance in Granular Magnetic Films,xe2x80x9d Physical Review B., Vol. 53, No. 16, at 927, and Miyazaki et al., xe2x80x9cSpin Polarized Tunneling in Ferro Magnet/Insulator/Ferro Magnet Junctions,xe2x80x9d Journal of Magnetism and Magnetic Materials,xe2x80x9d 151, at 403 in support of the explanation. However, the GMR effect in the Cheong et al. material can only be obtained at cryogenic temperatures which makes it unsuitable for the large scaled applications, such as the ones mentioned above.
Accordingly, what is needed is a GMR material which can be made in bulk and which exhibits a high magnetoresistance ratio at normal ambient temperatures, e.g., room temperature.
The invention provides a device comprising a magnetoresistive material that includes a first metallic ferromagnetic powder material at least partially coated with a thin insulating material and a second metallic ferromagnetic material in contact with the granules of the first metallic ferromagnetic powder material, the granules of the first ferromagnetic powder material having a higher coercive field than the second ferromagnetic material. According to an exemplary embodiment of the present invention, the granules of the first metallic ferromagnetic material are relatively hard, and the second metallic ferromagnetic material is also in powder form having granules which are relatively soft, the first and second metallic ferromagnetic powder materials being mixed and compressed to form a compacted mixture. According to a still further exemplary embodiment of the present invention, the first metallic ferromagnetic powder material has CrO2 granules at least partially coated with a thin layer of Cr2O3, and the second metallic ferromagnetic powder material has Ni granules.
Another aspect of the present invention is a method for making a device comprising a magnetoresistive material including the steps of mixing a first metallic ferromagnetic powder material having relatively hard granules at least partially coated with a thin insulating material, and a second metallic ferromagnetic powder material having relatively soft granules, the granules of the first ferromagnetic powder material having a higher coercive field than the granules of the second ferromagnetic powder material, and compressing, rolling or extruding the mixture to form a compacted magnetoresistive material. The magnetoresistive material in the device of the invention typically exhibits a magnetoresistance ratio of 8% at room temperature (i.e., 300xc2x0 K) in a magnetic field of about 4000 Oe applied substantially perpendicular to the direction of current flow in the material and a magnetoresistance ratio of 12% under the same conditions except that the direction of the applied magnetic field is parallel to the direction of current flow in the material.